1. Field of the Invention
This invention relates generally to camouflage techniques, and more particularly to real time active spectral matching camouflage.
2. Description of the Related Art
Camouflage is a technique for concealment, which involves disguising an object, in plain sight, in order to hide it from something or someone. Historically, camouflage has been designed to match colors, contours, and/or patterns of the camouflage to a background. For example, in times of conflict soldiers often wear special camouflage clothing to become less visible. Such camouflage may be placed between the viewer and the concealed object, or may be applied directly to the object itself.
Another type of camouflage is the creation of decoys. Most camouflage is designed to make an object appear to blend with its surroundings. In the case of decoys, the purpose of the camouflage is not to make the object “disappear” into the surroundings, but instead to make the camouflaged object appear to be another type of object. For example, in England prior to the D-Day invasion fake plywood “planes” were utilized to create the illusion of a large air force. Thus, it should be noted that “decoys” are a type of camouflage and are subsumed in the general category of camouflage.
Traditionally, military camouflage had to match the background only well enough to fool the human eye. As a result, military camouflage was only required to provide a good match over the narrow segment of the electromagnetic spectrum visible to the human eye. Hence, a few generic types of camouflage such as “Desert,” “Jungle,” or “Forest” were often sufficient to deceive human observers.
However, traditional camouflage techniques suffer from a number of basic defects. First, traditional camouflage is not adaptable to changes in the surroundings, requiring a different type of traditional camouflage for each different type of background. Second, traditional camouflage is passive in nature. That is, the signals that traditional camouflage returns to the observer are a function of angle of incidence, the source of illumination, and the composition of materials in the camouflage. Consequently, changes in conditions such as angle or amount of illumination may make the camouflage far less effective. Third, the reflective spectrum of traditional camouflage generally is not a good match to the surroundings. Before the advent of multispectral and hyperspectral remote-sensing techniques, matching the electromagnetic spectrum of the surroundings within the resolution of the human eye was sufficient. Today, with modern remote-sensing tools, differences between the spectrum of the surroundings and the spectrum of the camouflage are readily detectable even when undetectable to the human eye.
Many approaches have been proposed or implemented to address some of these defects in traditional camouflage systems. Most advanced military powers are well aware of the power of remote sensing, and have taken steps to improve their camouflage to minimize detectability across a wider spectrum. For example, U.S. Pat. Nos. 4,142,015 and 4,156,033 issued to Bienz (1979) provide a means of providing a varying degree of insulation to conceal radiation in the infrared region of the spectrum. U.S. Pat. No. 4,615,921 issued to Johanssen (1986) describes a plastic film that is matched to the thermal characteristics of particular surroundings. U.S. Pat. No. 5,077,101 issued to Conway, et al, (1991) describes a three-layer system designed to have different reflectances in different ranges of the spectrum.
These and other approaches to providing a better match to a given background all increase the concealing power of camouflage. None of these approaches resolves the fundamental problems cited above, however. That is, all of these approaches require a different camouflage for different backgrounds; are not active in nature; and are still unlikely to provide a tight enough spectral match to fool multispectral or hyperspectral remote-sensing instruments.
The problem of passive response to changing angles of incidence or changes in the assumed position of the observer has been addressed by U.S. Pat. No. 5,734,495 issued to Friedman (1998). In Friedman, the camouflage includes at least one set of movable surfaces that may be rotated to change the angle of incidence or expose increased surfaces of different camouflage materials with different reflective properties. This approach offers a higher degree of flexibility in dealing with the angle of the viewer. It also allows the spectral signature of the sum total of the camouflage to be adjusted to provide better matches to the surroundings. Unfortunately, the spectral signature must be constructed out of the limited palette of reflective surfaces that are built into the camouflage system.
Further approaches projecting light around an object such that what is behind the object is seen by an observer from the front. The first of these, U.S. Pat. No. 5,220,631 issued to Grippin (1993), employs fiber-optic cables and lenses to receive light from behind the concealed object and pipe it to the front of the object. Such a device might be fairly effective in deceiving a human observer, but a number of characteristics of the system would make it easily observable to instruments measuring spectral properties. Fiber-optic cables themselves, being composed of glass or plastic, have readily detectable spectral signatures. Although fiber-optics have excellent clarity, there is a degree of differential transmission of wavelengths of light through a fiber, so that the spectrum of the background after coming through the cables is likely to be measurably different from the spectrum of the surroundings. Finally, the approach is limited to the range of light energy readily transmitted by fiber optics; many of the devices used to detect camouflaged objects operate outside this wavelength range.
The second approach is that of U.S. Pat. No. 5,307,162 issued to Schowengerdt (1992). Schowengerdt discloses a cloaking system using optoelectronically controlled camouflage that interposes a shield between the viewer and the concealed object. On the outward surface of the shield is a “nonspecular display surface” that projects an image of the background (or any other desired image). This display screen is fed by a “fiber optic data bus.” The Schowengerdt reference suffers from all of the limitations of U.S. Pat. No. 5,220,631 cited above. Moreover, the Schowengerdt reference concedes that it is designed to operate in the visible light spectrum, and would therefore not provide concealment from standard military remote sensing instruments (or radar). In addition, the stated purpose of the patent is to provide an “image” rather than to match the detailed spectral properties for which camouflage-detection instruments search.
In view of the forgoing, there is a need for a camouflage system that provides a fully adaptive technique across the broad spectral range typical of current camouflage-detecting sensors. The camouflage system should be adaptable to changes in the surroundings, and active in nature. Further, the reflective spectrum of the camouflage system should match the reflective spectrum of the surrounding area.